1. Field of the Invention
The present invention relates to a piston, a method of producing the piston, and a pump having the piston.
2. Description of the Prior Art
Among pumps, a pump is conventionally known in which a diaphragm formed by rubber is used as a piston that is movably supported in a cylinder to be reciprocally driven. In the diaphragm serving as a piston, a closed end portion of the diaphragm is attached to a piston support member which is movably supported in the cylinder to be reciprocally driven while disposing a gap between the member and the inner face of the cylinder, and a cylindrical outer peripheral portion of the diaphragm is attached to the cylinder by a flange portion which is provided in an open end portion of the outer peripheral portion via an approximately 180-degree folded-back portion. In a pump comprising such a piston, suction and discharge ports for a liquid are opened in a pressure chamber which is defined in the cylinder by the piston, and a liquid tank and a discharged-liquid supplying portion are connected to the suction and discharge ports via check valves, respectively. When the pump is driven, the cylindrical outer peripheral portion of the piston rolls in the gap between the outer face of the piston support member and the inner face of the cylinder in accordance with the reciprocal movement of the piston support member, while the cylindrical outer peripheral portion is closely contacted with the two faces, whereby the volume of the pressure chamber is changed so that the liquid is sucked from the suction port in a suction step in which the volume expands, and the liquid is discharged from the discharge port in a discharge step in which the volume contracts (for example, see Japanese Patent Application Laying-Open No. 61-197779).
However, a conventional piston made of rubber is impaired by a corrosive liquid which attacks rubber, for example, a chemical solution such as a resist solution that is to be used in a process for producing an FPD such as an LCD, or a semiconductor device. Hence, such a piston cannot be used in a pump for supplying a corrosive liquid.
Therefore, a piston which can be used in a pump for supplying a corrosive liquid has been proposed (for example, see Japanese Patent Application Laying-Open No. 9-53566). This piston is formed by a fluororesin such as PTFE (polytetrafluoroethylen) which is resistant to a corrosive liquid, and is not impaired by a corrosive liquid. Unlike rubber, however, a fluororesin is not flexible and stretchable. Therefore, the section shape of a cylindrical outer peripheral portion of the piston is not circular but polygonal, so that bent portions are formed. Consequently, cracks, holes, or the like are formed in the piston, and the piston has a problem in durability.
The following technique has been proposed as means for improving the durability of a piston made of a fluororesin (see Japanese Patent Application Laying-Open No. 9-53566). In the technique, a tapered portion which is more thinned as further advancing toward a pressure chamber is disposed on the outer face of a piston support member. Furthermore, an inclination angle of a cylindrical outer peripheral portion of the piston and directed toward a driving chamber (which is defined on the side opposite to the pressure chamber across the piston), and that of the tapered portion and directed toward the pressure chamber are set to be substantially equal to each other. In the means, however, the cylindrical outer peripheral portion of the piston can be closely contacted with the outer face of the piston support member, but not with the inner face of a cylinder, and a gap is formed between the outer peripheral portion and the cylinder inner face. Therefore, undulating deflections or bends are produced in the cylindrical outer peripheral portion of the piston, and cracks, holes, or the like are formed in the piston.